1. Field of the Invention
The present invention relates to a method for accurately measuring each position of optical transmission members such as optical fibers and the like in a retainer holding these optical transmission members.
2. Description of Related Art
There are known various substrates for fixing optical fibers with a diameter of, for example, about 125 .mu.m. In any substrates, if an optical axis of the optical fiber fixed to the substrate is deviated from a given position, a transmission loss between the optical fiber and another optical transmission means becomes large. Therefore, when grooves for fixing the optical fibers are formed on the substrate, it is required to have a very high working accuracy of, for example, not more than 0.5 .mu.m. In such a substrate, there is adopted, for example, a method wherein grooves of V-shaped section are formed on the substrate, and optical fibers are placed in the respective grooves and then the optical fiber is fixed in the groove with a resin or a solder.
FIG. 1(a) is a front view schematically showing an embodiment of the optical fiber retainer provided with the above substrate for the fixation of the optical fibers, and FIG. 1(b) is a plan view schematically showing this retainer, and FIG. 1(c) is a side view of the retainer viewed from left side of FIG. 1(b). The optical fiber retainer 1 comprises a substrate 3 for fixing the optical fibers. The substrate 3 is comprised of a stepped portion 3a and a holding portion 3b, in which a covering 2 of optical fibers is fixed to the stepped portion 3a and a given row number of grooves 7 are formed on the holding portion 3b. Each of the grooves 7 extends from an end face 6 of the holding portion 3b toward the stepped portion 3a. An optical fiber 8 is placed into the groove 7 and then fixed thereto with a resin or the like. A lid 4 is put over the holding portion 3b, and a lid 5 is put over the covering 2.
There will be described a method for manufacturing such an optical fiber array. At first, there is provided the substrate 3 for fixing the optical fibers, which may be made from ceramic or glass. Each of the grooves 7 is a fine groove having a depth of, for example, about one hundred several tens .mu.m. The optical fibers 8 are individually placed and aligned in the grooves 7.
Since the optical fiber array is used to couple with an optical waveguide substrate or the like, it is necessary to accurately measure each position of the optical fibers. Because the optical waveguide can be formed with an accuracy of .+-.0.2 .mu.m, for example, by a semiconductor process. Assuming that optical fibers are coupled with optical waveguides having the same profile as the optical fiber among various optical waveguides, when a deviation between an optical center of the optical fiber and that of the optical waveguide is 1.5 .mu.m, a coupling loss of not less than 0.4 dB is caused. Since the coupling loss is desired to be not more than 0.2 dB, the deviation between both the optical centers is required to be not more than 1.0 .mu.m. For this purpose, it is required to measure the deviation of the core position of the optical fiber from an ideal or designed position with a measuring accuracy of .+-.0.1-0.2 .mu.m.
Nevertheless, it was impossible to measure the core position of the optical fiber with such a high accuracy. Namely, the shape of the groove accommodating the optical fiber can be measured by means of a contact type shape measuring device, but the position of the optical fiber can not directly be measured by such a device. This is due to the fact that since an end face of the optical fiber is machined to be flat by polishing, the position of the optical fiber can not be known even when the contact type shape measuring device is applied to the end face of the optical fiber array. If the optical fiber completely contacts with the groove, it is possible to calculate the fixed position of the optical fiber from the position and shape of the groove with a high accuracy. However, the optical fiber is actually in contact with the groove wall surface at a slight clearance, so that it is impossible to calculate the position of the optical fiber with the aforementioned high accuracy.
In order to solve this problem, the present inventors considered to measure the core position of the optical fiber by means of a universal tool microscope. In the universal tool microscope, a work is placed on a movable stage, so that a coordinate is indicated in the positioning through the microscope. However, in conventional universal tool microscope, the measuring accuracy is only about .+-.1 .mu.m. Judging from the measuring accuracy of the stage itself, it is difficult to more improve the measuring accuracy over the above value.
Furthermore, the present inventors considered that a laser beam is passed through each optical fiber to receive a beam transmitted from the optical fiber with CCD and the optical center of the optical fiber is measured from the received beam. However, it is very difficult to measure a profile of the beam. Particularly, when the beam transmitted from the optical fiber is directly observed, a beam disordered from a circular shape is seen due to the problem of a pixel accuracy of CCD and the like. In order to determine the center of this beam, it is necessary to calculate the optical center by normalizing the profile of non-circular beam, but there is actually a limit in the measuring accuracy. In case of using a commercially available beam profiler, the measuring accuracy was only about .+-.10 .mu.m. Moreover, the measuring accuracy may theoretically be enhanced to a higher level by trial-manufacturing a high accuracy lens system and a stage. In the latter case, however, the measuring accuracy is 0.5 .mu.m at most.